The thick filament is an organized permanent assemblage of myosin molecules in muscles. Vertebrate thick filaments contain C protein, titin, and M band proteins that associate with myosin whereas non-vertebrate filaments contain the rodlike protein paramyosin in the filament core as well as the associated twitchin/projectin homologues of titin/C protein. Because of extensive developmental, genetic, molecular and cellular and biochemical studies, the thick filaments of Caenorhabditis elegans are proposed as a non-vertebrate model for this important macromolecular assembly. A preliminary negative stain reconstruction of the core assembly shows rod-like features surrounding, and cross-linked by, a central tube containing other globular density, perhaps representing other proteins known to associate with the core. Both the core and intact thick filaments are being investigated by cryomicroscopy. In a study of the cores published this year, we have proposed a model for the core and its associated internal proteins. The model fits best with the observed calculated Fourier transform when the paramyosin is arranged as 7 strands around the outside of the core, and the internal proteins are distributed across the gap of single-stranded paramyosin in an asymmetric fashion. Furthermore, the diffraction pattern is sensitive to the fraction of the length occupied by these internal cores, with 25% of the length yielding the best fit.